The present invention relates to a spar buoy construction having a production deck, oil storage facilitates, and a riser system connecting a subsea well installation on the sea floor with the production deck, the riser system extending through a central longitudinal passageway in the spar buoy hull means. The spar buoy construction is adapted to operate in water depths of from 1,000 to at least 7,000 feet and under conditions of severe exposure.
Prior proposed offshore constructions have included jacket type structures fixed to and resting upon the sea floor, a guyed tower construction such as shown in U.S. Pat. No. 3,903,705 and further described in offshore magazine of April 1983 pages 47-60 inclusive, and tension mooring platforms such as shown in U.S. Pat. Nos. 3,780,685 and 3,648,638. Offshore oil storage capacity is desired at at least a minimum of 500,000 barrels and preferably from 750,000 to 1,000,000 barrels. It does not appear feasible to provide oil storage of over 100,000 tons in association with the jacket type, guyed tower, and tension mooring platform constructions briefly mentioned above. The present objective is to provide an offshore floating construction which will include integrated crude oil storage at reasonable costs combined with production facilities and with a riser system associated with the floating oil storage structure. One of the advantages of a spar buoy construction for this objective is that the natural period in heave, pitch, and roll motion may be made longer than the period of an expected ocean wave. Motion of a spar buoy construction may be made less than motion of a semi-submersible or of a floating vessel of generally horizontal profile.
An important parameter to be considered in offshore structures of this spar buoy type is the distance between the center of gravity of the entire structure and the center buoyancy of the structure. It is necessary for the center of gravity to be below the center of buoyancy for stability. If the distance between the gravity and buoyancy centers is long then the structure will assume a short natural period in pitch and roll and the dynamic motion response to waves will be large. If the buoyancy center to gravity center is a short distance, the structure will assume greater tilt or inclination in response to wind and current conditions but have a reduced response to wave conditions. An optimum or a preferred distance between the center of gravity and center of buoyancy of a structure may be determined and if such center of buoyancy to center of gravity distance is maintained constant, motion stability of the structure will be enhanced.
Another parameter considered involves riser systems of more than a 1,000 feet in depth. Such riser systems are subjected to substantial horizontal loading and require either some form of lateral support or tensioning. In the tension moored platform, simple tensioning of the riser system is possible. In a semi-submersible vessel heave compensators are necessary to maintain constant tension in the risers.
Floating oil storage units of spar buoy type are shown in U.S. Pat. Nos. 3,921,557 and 3,360,810. The latter patent shows an external riser system having a vertically elongated hull for oil storage and adjacent the top thereof side tanks to provide variable balance ballast to maintain a constant draft of the vessel. In U.S. Pat. No. 3,921,557 a flexible riser means extends through a central passageway of a spar buoy type hull, the riser being provided with tensioning means and the hull providing storage for oil.
Various types of offshore oil storage vessels have been proposed such as shown in U.S. Pat. Nos. 3,507,238; 3,880,102; 3,889,477; 3,837,310; 4,059,065. In general the oil storage structures of these patents seeks to maintain constant weight during filling and discharge of oil by varying the water ballast to maintain generally constant draft conditions for the vessel.
In U.S. Pat. No. 3,470,836 a subsea well head structure provides an elongated hull not used for oil storage but to provide a central passageway through which a riser extends, the riser having a float to maintain the riser under suitable tension. The hull has a top submerged work chamber and is normally located entirely below the surface of the water.
In general offshore structures have been designed specifically for one or two functions such as storage of oil or drilling and production facilities with oil storage being separate from such facilitates.